This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Simpson states that he has often seen benefit from inhalations containing 5 to 10 per cent. of carbonic acid in these maladies, and in chronic cough. Such inhalations are much better tolerated than is commonly thought (Skinner), and they are practised at St. Moritz, at Ems, and elsewhere, but Dr. C. T. Williams speaks of danger arising from them, on account of difficulty in regulating the dose (Lancet ii., 1873, p. 516). The relief given in asthmatic attacks by the fumes of nitre paper has been attributed to the carbonic acid contained in them, and it is a matter of clinical experience that asthmatic patients frequently breathe better in a crowded part of a town - where the amount of this gas is greater - than they do in the pure air of the country.